This invention relates to the recovery of useful materials from waste materials and more particularly to the treatment of sludge formed in tin plating baths to recover useful quantities of tin compounds and ferrocyanide compounds for recycling.
Among the commercial processes used to produce tin plated steel strip is the so-called Halogen Tin Electro-deposition process. In this process, pickled and washed steel strip is passed into a bath containing a complex stannous fluoride anion, thought to be SnF.sub.6.sup. 4.sup.- (hexafluorostannate(II)), sodium bifluoride, hydrochloric acid and minor amounts of various addition agents such as grain refiners, all as well known to those skilled in the art. The strip is made cathodic as it passes into the bath and a metallic tin anode is immersed in the bath. In operation, an electric current is applied causing the stannous fluoride complex ion to approach the cathodic steel strip surface where it is reduced to metallic tin, resulting in the deposition of a layer of tin on the steel strip.
One of the disadvantages of this halogen tin process is that the bath tends to accumulate an ever increasing amount of sludge during operation, the major component of which sludge is sodium hexafluorstannate(IV), Na.sub.2 SnF.sub.6, together with some hydrated stannic oxide, SnO.sub.2.xH.sub.2 O. The hexafluorostannate (IV) complex is formed by air oxidation of the hexafluorostannate(II) complex. This oxidation of stannate(II) to stannate(IV) results from the entrapment of air in the plating solution due to vigorous agitation of the plating bath caused by the high line speed of the strip. Although the oxidation is kinetically slow, the rate may be catalytically increased in the presence of dissolved iron, which is present in the plating bath due, at least in part, to incomplete washing of the strip following the acid pickle.
In order to minimize the amount of ferrous ion present in the electroplating bath, and, thereby, prevent catalysis of the hexafluorostannate(II)/hexafluorostannate(IV) oxidation, sodium ferrocyanide, Na.sub.4 [Fe(CN).sub.6 ].10H.sub.2 O, is periodically added to the bath. The ferrocyanide ion, Fe(CN).sub.6.sup. 4 .sup.-, has great affinity for ferrous ion, and will readily combine therewith to form a so called ferro-ferrocyanide complex ion which will precipitate from the bath solution as Fe.sub.2 Fe(CN).sub.6. This salt is, in turn, slowly oxidized by the air which is drawn into the bath, to ferriferrocyanide Fe.sub.4 [Fe(CN).sub.6 ].sub.3, which along with the Na.sub.2 SnF.sub.6 and SnO.sub.2.xH.sub.2 O then forms a portion of the bath sludge which must be periodically removed. Sodium ferrocyanide additions are then made periodically in order to replenish the ferrocyanide available to remove any ferrous ion present.
The bath sludge is of commercial value due to the presence of the high-grade tin source compound, Na.sub.2 SnF.sub.6, and a well known process is customarily used for its recovery. This process involves treating the sludge with a sufficient volume of hot water to leach the majority of the Na.sub.2 SnF.sub.6 from the sludge. The insoluble matter remaining, which is hereafter called secondary sludge, is removed from the hot water solution and, depending on the tin content, discarded or sold to a smelter. The clear solution remaining is then further processed to obtain a tin bearing compound or metallic tin.
Thus the presently used process for halogen tin sludge treatment fails to recover any of the ferrocyanide present in the halogen tin bath sludge. The failure to extract the ferrocyanide portion results in both an increased cost of the tin plating operation itself (due to a constant need for fresh sodium ferrocyanide) and a potential pollution problem due to the cyanide content of the secondary sludge which must be discarded.